This invention relates to a retaining wall construction, suitable for use in civil engineering soil reinforcement, of the type comprising a wall built from staggered superimposed courses of modular blocks anchored to a reinforcement material, preferaby a geogrid.
Reinforcement materials are well known in civil engineering construction work, to stabilise a reinforce large volumes of soil, such as embankments, terracing and landfill. They are usually lid horizontally between layers of compacted soil infill, with the vertical spacing between successive layers normally increasing from the bottom to the top of the infill. The reinforcement material can take many forms but is typically a mesh, grid, net or perforated sheet made from a non-biodegradable material, such as various plastics or metal wire, and in particular one of the woven or integral polymeric grids known as geogrids.
Although the present invention is not limited to the use of any specific type of reinforcement material, of particular interest are geogrids made by stretching a sheet of plastics material (such as high density polyethylene) having a pattern of holes formed therein, so as to produce a rectangular mesh with parallel spaced-apart molecularly-oriented strands interconnected by transverse bars. Such geogrids are described, for example, in the specifications of U.S. Pat. No. 4,374,798, British Patent 2 073 090 and British Patent 2 235 899, and are available commercially under the Trademark "TENSAR".
For brevity, the term "geogrid" will generally be used herein, to denote the reinforcement material employed in the invention. However, this term should be understood also to cover other forms of flexible strip or sheet-like material suitable for use in soil reinforcement, such as woven or non-woven textiles, webs or sheets, providing that these materials posess the strenghth and other properties needed for the intended use and are capable of interacting correctly with the other elements of the invention, as described below.
In many types of soil reinforcement construction, it is necessary to provide a retaining wall along at least one side of the infill, for instance to prevent erosion. Such a wall may be constructed from superimposed courses of loose-laid modular blocks, with staggered joins between the blocks in successive courses, in the conventional manner. The modular blocks may conveniently be pre-cast on or off site from unreinforced or mass concrete, preferably to a size allowing for easy handling without the use of cranes or other heavy lifting gear. The wall may be straight or curved along its length, by using blocks of an appropiate design, and may be vertical or with a batter (i.e. its face may slope backwards from bottom to top).
A retaining wall of this kind must be able to withstand the considerable pressure of of the soil infill behind it, and this can be done by anchoring it to the substantially horizontal geogrid material buried between the layers of the infill. As the construction proceeds, the wall is built up from courses of the modular blocks, an the soil infill is added behind it and compacted. Layers of the geogrid material are laid horizontally over the compacted soil, at appropiate vertical intervals, and anchored to the wall. The process is then repeated until the final height is achieved. The vertical spacing between the layers of geogrid is often greater than the height of the blocks, so that two or more courses of blocks will frequently be laid between successive layers of geogrid.
Various designs of retaining wall and methods for anchoring a geogrid to the retaining wall have been proposed in the past; but these have generally suffered from various disadvantages, such as not providing adequate strength of anchoring, not providing anchoring evenly along the major part of the interface between the geogrid and the wall, or not being readily usable with curved retaining walls.
Thus, in one type of construction, for example as described in European Patent Specification 0 472 993, the edge of the geogrid is simply trapped between two courses of blocks in the retaining wall, without any positive means of engagement between the geogrid and the wall blocks. This permits the construction of curved walls, by using suitably shaped blocks; but the retaining wall is anchored only by the strength of frictional forces between the geogrid and particles of the infill, and between the geogrid and the blocks, generated by the weight of the superimposed courses of blocks, which may be insufficient in many situations.
Other designs of blocks and forms of wall construction, such as described in U.S. Pat. No. 5,417,523 or PCT Publication WO94/13890, do provide a more secure form of attachment between the geogrid and the wall, but are suitable for use only in straight retaining walls and not in curved walls (except those with large radii of curvature), because their design does not allow for significant articulation between adjoining blocks laid within the courses of the wall. Such designs typically employ a bar-like retaining member with spaced-apart projections fox engagement with the apertures in the geogrid, with this retaining member being anchored in channels bridging the adjacent upper and lower faces of two courses of blocks in the wall, or formed in the rear vertical face of the wall blocks. Whilst these designs may provide a secure method for anchoring the geogrid to the wall evenly along its width, they also have the effect of interlocking the blocks in a substantially linear array and so cannot be used for the construction of sharply curved walls.
Similar problems arise with the wall block construction disclosed in the recently published U.S. Pat. No. 5,540,525, which uses a system of slat members keyed into slots at the intersections between the blocks to maintain vertical and horizontal alignment. This system also does not cater for the construction of stable walls with a significant degree of curvature.
Yet other alternative designs, such a described in U.S. Pat. Nos. 4,825,619 and 4,914,876, do allow for articulation between adjoining blocks and hence the construction of curved walls. However, these designs anchor the geogrid to the wall by means of rods inserted through vertically aligned bores in the superposed courses of blocks, which pass through single apertures in the edge of the geogrid inserted between the courses. The anchoring of the geogrid to the blocks is therefore concentrated at the point of contact with the rods and not distributed evenly, which limits the strength of the system and may result in distortion and failure of the geogrid. There are also other blocks, such as those described and shown in U.S. Pat. No. 5,505,034, which are of a shape suited to the construction of curved as well as straight retaining walls, but which make no specific provision at all for retaining a geogrid between the courses.
There is, therefore, a need to provide a method for securely attaching the geogrid to the retaining wall, which is versatile enough for use with the various types and forms of geogrid, and which can also be used with vertical or sloping, straight or curved retaining walls.